Block copolymers of vinylaromatics (eg. styrene) and dienes (eg. butadiene) are copolymers of a plurality of polymer molecular segments (ie. blocks) which are arranged in series or otherwise linked and have a more or less uniform composition. Depending on the structure and content of diene monomers, they may have overall--at a certain temperature--elastomeric properties or rigid, nonelastomeric properties, ie. as a whole they either exhibit elastomeric behavior similar to a polydiene and are important, for example, as SB rubber, or they behave as transparent, impact-resistant styrene polymers. Similarly to the definitions in the case of toughened polystyrene, it is usual to refer to those moieties which determine the elastomeric behavior as the soft phase and to the rigid moieties (the pure polystyrene fraction) as the hard phase. SB rubbers must be vulcanized in the same way as the usual diene polymers for use, which greatly restricts their use and makes processing more expensive.
The present invention relates to usually transparent block copolymers of vinylaromatics and dienes, which block copolymers can be processed by a purely thermoplastic method and have elastomeric behavior and particular mechanical and improved thermal properties.
The following must thus be mentioned in this context:
The anionic polymerization which leads to living polymers and in which the growth of a chain molecule takes place at a chain end which, in the absence of a spontaneous chain termination or transfer reaction, theoretically lives for an unlimited time (remains polymerizable), and the reaction of the living polymer with monofunctional or polyfunctional reactants is known to be a versatile possible method for synthesizing block copolymers, although the choice of monomers is limited; in practice, only block copolymers of vinylaromatic compounds, ie. styrene and its derivatives on the one hand and dienes, essentially butadiene or isoprene, on the other hand, have become important. Block copolymers are obtained by in each case carrying out polymerization until a monomer stock is virtually exhausted and then changing the monomer or monomers. This process can be repeated several times.
Linear block copolymers are described, for example, in U.S. Pat. Nos. 3,507,934 and 4,122,134. Star block copolymers are disclosed, for example, in U.S. Pat. Nos. 4,086,298; 4,167,545 and 3,639,517.
The property profile of these block copolymers is essentially characterized by the content of polymerized diene monomers, ie. the length, arrangement and ratio of polydiene and polystyrene blocks. Moreover, the type of transition between different blocks plays an important role. The influence of crisp and tapered transitions (depending on whether the monomer change is abrupt or gradual) is explained in detail in DE-A1-44 20 952, so that further description is unnecessary here.
It is merely necessary to point out that, in block copolymers having a tapered block transition, the sequence lengths are by no means randomly distributed but that the sequence length of the pure diene phase compared with the polystyrene phase and hence the volume ratio are shifted in favor of the diene phase. This has the disadvantage that the poor properties of the diene polymer are unnecessarily strongly evident in the material behavior during processing.
In particular, materials having a diene content of more than 35% by weight, which, owing to their property profile (toughness, transparency, gas permeability), would be suitable for medical applications, such as infusion tubes, infusion drip chambers and stretch films, can be processed by profile extrusion, injection molding or tubular film extrusion only with very great difficulty; in spite of stabilization with antioxidants and free radical acceptors, they are very heat-sensitive and tend to become tacky, necessitating an expensive remedy with additives. Blocking (sticking of films and tubes to the roll) and poor mold release properties may make processing by injection molding completely impossible.
DE-A1-44 20 952 has therefore proposed in this context the preparation of an elastomeric block copolymer which consists of at least one block A having polymerized units of a vinylaromatic monomer and forming a hard phase and/or a block B containing diene monomers and forming a first elastomeric (soft) phase and at least one elastomeric block (B/A) having polymerized units of a vinylaromatic monomer and of a diene and forming a soft phase or a further soft phase, the glass transition temperature T.sub.g of block A being above 25.degree. C. and that of block (B/A) being below 25.degree. C. and the phase volume ratio of block A to block (B/A) being chosen so that the proportion of the hard phase in the total block copolymer is 1-40% by volume and the amount of the diene is less than 50% by weight. These block copolymers already constitute a considerable advance over the previously known block copolymers having tapered block transitions. However, materials having a diene content of up to 35% also tend to form gels (crosslinking Via the olefinically unsaturated chain elements) when subjected to prolonged thermal stress and shear stress, as occur in particular during extrusion. Particularly in the production of films, gel particles may be evident as troublesome specks. The tendency to crosslink is ascribed in particular to the short chain branches present in polydienes, ie. side chains having the structure ##STR1##